The formation of intimate, atomically clean, In-In80Al20Sb interfaces has been investigated using x-ray photoelectron spectroscopy. The attenuation of the substrate (Sb) core levels showed anomalous behavior; increasing for the first few monolayers of In deposited. This is interpreted in terms of a reaction between chemisorbed In on the c(8X2) reconstructed In-rich In80Al20Sb surface and deposited In, to produce clustering, so enhancing the substrate signals emanating from bulk In80Al20Sb. The incremental deposition of In brought about incremental shifts of the In core level binding energies, related to changes in the chemical environment of the In. For the other bulk core levels examined, no further shifts were resolved, indicating that the Fermi level at the In80Al20Sb surface is pinned prior to metallization.